1. Field of the Invention
The present invention relates to a permanent magnet type stepping motor, and more particularly a permanent magnet type stepping motor which comprises a stator, two rotor poles and a permanent magnet, wherein the stator has a plurality of magnetic poles provided radially on its inside periphery, a plurality of pole teeth are provided on the tip of each pole with an identical pitch, windings are wound on respective magentic poles, pole teeth with the same pitch as the pole teeth of the stator are provided on the whole peripheries of the two rotor poles, and the permanent magnet is magnetized in the direction of a shaft and held by the two rotor poles.
2. Description of the Prior Art
An embodiment of a permanent magnet type stepping motor of the prior art is shown in FIGS. 1(a) and 1(b). In FIGS. 1(a) and 1(b), the reference numeral 1 denotes a stator housing; 2, a stator core; 3, stator windings; and 4 and 4, end brackets.
A plurality (in this example, 8) of magnetic poles 2-1-2-8 are provided on the internal periphery of the stator core 2 and a plurality of pole teeth 2-10 are provided on the tip of each pole with an identical pitch and windings 3-1-3-8 are wound on respective magnetic poles.
Pole teeth 7-10 and 8-10 are provided on the whole peripheries of rotor poles 7 and 8 with the same pitch as the pole teeth 2-10 on the stator poles and positions of the respective pole teeth of the rotor poles 7 and 8 are shifted by 180.degree. to each other. The rotor poles 7 and 8 are attached solidly to a rotor shaft 6 while holding a permanent magnet 9. The pole teeth 7-10 and 8-10 of the rotor are held to face the pole teeth 2-10 of the stator by bearings 5 and 5 so as to be able to rotate freely.
Normally, stator windings on the poles of the positions symmetrical to the shaft are connected in series to form four sets of windings. By applying current to the respective windings successively, the magnetic pole, to the winding of which the current is applied, is magnetized and facing pole teeth of the rotor are attracted by the pole teeth of the magnetized stator pole to generate torque. When the pole teeth of the rotor reach the position where the pole teeth of the rotor and the pole teeth of the stator are aligned, the torque is lost and the rotation is discontinued.
If the current is applied to the windings 3-1 and 3-5 in FIG. 1(b), the magnetic poles 2-1 and 2-5 on which those windings are wound are magnetized. If the pole teeth of the rotor poles 7 and 8 are aligned with the pole teeth of the respective magnetic poles, pole teeth provided on the tips of magnetic poles 2-2 and 2-8 which are adjacent to the pole 2-1; 2-4 and 2-6 which are adjacent to the pole 2-5 and 2-3 and 2-7 which are apart from the poles 2-1 and 2-5 are not aligned with the pole teeth of the rotor at the facing position but shifted by a certain theoretical angle. In other words, the pole teeth provided on the magnetic poles 2-2 and 2-6 are shifted against the facing pole teeth of the rotor by an angle corresponding to 1/4 of the pitch of the pole teeth of the rotor to the clockwise direction; the pole teeth provided on the magnetic poles 2-3 and 2-7 are shifted against the facing pole teeth of the rotor by an angle corresponding to 2/4 of the pitch of the pole teeth of the rotor to the clockwise direction; and the pole teeth provided on the magnetic poles 2-4 and 2-8 are shifted against the facing pole teeth of the rotor by an angle corresponding to 3/4 of the pitch of the pole teeth of the rotor to the clockwise direction. Therefore, if the current application is discontinued and the current is applied to the windings 3-2 and 3-6 when the rotor is at the position described above, the rotor rotates to the right by the angle corresponding to 1/4 of the pitch of the pole teeth of the rotor and stops. This angle of rotation is called a step angle of this stepping motor and is a specific value which is 1/4 of the pitch of the pole teeth of the rotor.
In order to make the rotor rotate further right, the current is applied to the windings 3-3 and 3-7 to magnetize the right side adjacent magnetic poles 2-3 and 2-7. In the same way, in order to make the rotor rotate to the left, the current is applied to the windings of the magnetic poles adjacent to the left of the poles excited at present. The number of switchings of the windings to which the current is applied is proportional to the angle of rotation and the direction of the rotation can be controlled by switching the current-applied windings to the right or to the left. Therefore, angle, speed and direction can be controlled by a simple controller; consequently, a large number of stepping motors have been employed as control motors.
In order to increase resolution, one of the control functions of a stepping motor, usually the number of pole teeth is increased. For instance, in the embodiment of FIGS. 1(a), 1(b), a stepping motor with one step angle of 1.8.degree. is realized by providing 50 pole teeth on both rotor poles 7 and 8 and providing 5 pole teeth on each tip of 8 poles of the stator.
However, such stepping motors as described above have increasingly been applied to magnetic disc driving apparatuses employed as external memory apparatuses of electronic computers and have been used as the driving force of magnetic heads of the magnetic disc driving apparatuses and step angles of the stepping motors have been matched to the pitch of record tracks of the magnetic discs. Recently, an apparatus which enables recording more data on one disc by reducing width of the tracks to 1/2 or 1/4 of their previous width has been developed, and accordingly, the step angle of a stepping motor has been required to be reduced to 1/2 or 1/4 of the previous angle.
In principle, resolution could be infinitely improved by increasing the number of pole teeth provided on the rotor. However, as there was a limitation of the dimensions of the motor, and there was also a limitation of processing teeth pitch for increasing the number of pole teeth keeping the diameter of the rotor almost equal to the conventional one, consequently, 0.9.degree., a half of 1.8.degree. was the minimum.